A known hybrid automatic repeat request (HARQ) is the standard of a packet retransmission process performed between base stations and mobile terminals. A HARQ is used in, for example, Long Term Evolution (LTE), which is a standard set by the 3rd Generation Partnership Project (3GPP). In the following, transmitters and receiving apparatuses used in a mobile communication system that conform to LTE will be described.
In mobile communication systems that conform to LTE, a transmitter adds an error detection code to data to be transmitted. Furthermore, the transmitter splits the data if the size of the subject data is greater than a predetermined size. In the description below, the split data is sometimes referred to as a “code block”. After the transmitter adds, for each code block, the error detection code, the transmitter encodes the code blocks and adds a redundancy code to them. Then, the transmitter couples the encoded code blocks. And then, the transmitter performs, for example, a modulation process on the coupled data stream and transmits the modulated signal to the outside. In the description below, the data to be transmitted by the transmitter is sometimes referred to as a “transport block”.
One of the reasons for splitting the transport blocks is that the memory size used by encoders or decoders increases as the size of data to be encoded increases. Accordingly, in general, the size of the data to be encoded is sometimes limited with aim of suppressing any increase in memory size. For example, with LTE, if a turbo code is used as a redundancy code, the upper limit of the size of the data to be encoded is determined to be 6,144 bits.
Furthermore, in mobile communication systems that conform to LTE, if a signal received from the transmitter is greater than a predetermined size, a receiving apparatus splits the received signal into a code block. At this time, the receiving apparatus retains the signal that has been split (hereinafter, referred to as a “split signal”) in a predetermined buffer. In the description below, the split signal that is retained in the predetermined buffer described above is sometimes referred to as a “retained signal”.
Then, the receiving apparatus performs error correction or decoding on the retained signal and performs error checking on code blocks that are created by the decoding. Then, the receiving apparatus couples the code blocks to create transport blocks and performs the error checking on the transport blocks. If the receiving apparatus detects an error in a transport block, the receiving apparatus sends a request to the transmitter to retransmit the signal. Specifically, the receiving apparatus sends the retransmission request in transport block units. In the description below, if a signal is transmitted to a predetermined signal, the predetermined signal is sometimes referred to as a “retransmission source signal” and the signal that is retransmitted is sometimes referred to as a “retransmission signal”.
Furthermore, if the receiving apparatus receives a retransmission signal, the receiving apparatus uses maximum ratio combining to combine the split signal, which is obtained by splitting the retransmission signal, and the retained signal, which is retained in the predetermined buffer. At this time, the receiving apparatus overwrites the combined signal, which is created by being subjected to the maximum ratio combining, in the predetermined buffer. Accordingly, if the receiving apparatus receives the retransmission signal, the combined signal becomes the new retained signal. Then, the receiving apparatus performs error correction, decoding, or error checking on the retained signal subjected to the maximum ratio combining and couples the code blocks that are created by the decoding. Then, the receiving apparatus performs the error checking on the coupled transport blocks. If the receiving apparatus detects an error, the receiving apparatus again sends a request to retransmit the signal to the transmitter.
As described above, because a mobile communication system, such as an LTE mobile communication system, that uses a HARQ uses maximum ratio combining to combine the retransmission signal and the retained signal, the signal to noise ratio (SNR) is high.
Patent Document 1: Japanese Laid-open Patent Publication No. 2003-008553
Patent Document 2: Japanese Laid-open Patent Publication No. 2008-067171
Patent Document 3: Japanese Laid-open Patent Publication No. 2002-111637
However, in the conventional technology described above, there is a problem in that the throughput of the system decreases. In the following, the problem will be described.
The mobile communication systems of the conventional technology described above that use a HARQ perform a retransmission process on signals in transport block units. When the receiving apparatus receives a retransmission signal, the receiving apparatus uses maximum ratio combining to combine the retained signal and the retransmission signal even if an error is not detected in the retained signal. Accordingly, if the receiving apparatus receives, for example, a retransmission signal that contains an error, the receiving apparatus may possibly use the maximum ratio combining to combine the retained signal, in which an error has not been detected, and the retransmission signal, which contains an error. If a signal subjected to the maximum ratio combining in this way contains an error, the receiving apparatus detects an error at the time of error checking performed on a transport block and again sends a request to retransmit the signal to the transmitter. Specifically, the receiving apparatus may possibly send the request to retransmit the signal because the retransmission signal contains an error even though the retained signal does not contain an error. Accordingly, with the conventional mobile communication system described above, throughput may decrease.
To solve the above problem, it is conceivable to perform a retransmission process for each code block; however, with such a method, because the number of retransmission requests transmitted from the receiving apparatus increases, throughput consequently decreases.